The oral vaccine -- which contains weakened live virus –- is cheap, easy to give, and extremely effective. But the viruses may genetically alter in an immunized person’s gut and revert to causing disease. In 2012, more countries reported cases of this "circulating vaccine-derived poliovirus" (cVDPV) than wild poliovirus (WPV).

The original plan was for every country to stop using live vaccine simultaneously when wild polio disappeared, switching instead to a killed polio vaccine that would protect children as any lingering cVDPV died out. But if wild polio or cVDPV return after that, countries will need intense surveillance to spot it and live vaccine to contain the spread, fast.

Countries will switch to killed vaccine before eradication is over. (The wild virus, WHO predicts, will largely be gone by 2015.)

At that point, the 140 countries most at risk of resurgence will start giving killed vaccine and live vaccine, but one that’s effective against only two of polio's three strains. (Nearly all cVDPV is type 2, which was eradicated in 1999.)

By using live vaccine containing only types 1 and 3, countries can maintain immunity to any persisting wild polio, while cutting off the source of cVDPV.

And it must be done all at once, because if one country stops using type 2 live vaccine, while a neighbor continues, the first is at risk from type 2 cVDPV.

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